1. Field of the Invention
This invention relates to a liquid crystal display apparatus, and more particularly to a liquid crystal display apparatus in which twisted nematic liquid crystals having different orientation directions are provided in one picture element.
2. Description of the Prior Art
A liquid crystal display apparatus is employed in a broad field of OA equipments, television receivers, vehicle-carried equipments and so forth because of its characteristics of low power consumption, thinness, and lightness in weight. In the usual liquid crystal display panels employed at present, the upper and lower substrates constituting a panel are processed by orientation processing that gives the orientation direction of the liquid crystal filled between the substrates a predetermined tilt angle. A liquid crystal display panel of this type provides particularly narrow angles of visibility in the direction in which the liquid crystal is tilted and in the opposite direction, become of the asymmetry in orientation of the liquid crystal itself.
As a technique for canceling the asymmetry in orientation to assure a wide angle of visibility for a liquid crystal panel, an orientation dividing method has been proposed wherein, as shown in FIG. 1(a), a display picture element of a liquid crystal panel including TFT substrate 1 and opposed electrode substrate 2 between which liquid crystal is filled is processed by rubbing in different directions to divide it into two areas: normal tilt area 12 (hereinafter referred to as A) and reverse tilt area 13 (hereinafter referred to as B) which have different liquid crystal orientation directions (for example, SID '93 Digest, p.265).
A reason why the visibility angle characteristic of an orientation dividing panel formed based on the orientation dividing method described above is improved is described below.
FIG. 1(b) is a diagram showing voltage-transmission factor characteristics of liquid crystal having the orientation direction of A and liquid crystal having the orientation direction of B. In particular, shown in FIG. 1(b) are characteristics in three directions including characteristics (curves of A,B.+-.0.degree.) when the liquid crystal is viewed from the front, characteristics (curves of A+30.degree. and B+30.degree.) when the crystal is viewed from the direction of 30.degree. above, characteristics (curves of A-30.degree. and B-30.degree.) when the crystal is viewed from the direction of 30.degree. below, and characteristics (curves of C.+-.0.degree. and C.+-.30.degree.) of averages of the characteristics of the crystals in the individual directions. In FIG. 1(b), the curves of A,B.+-.0.degree. and C.+-.0.degree., the curves of A+30.degree. and B-30.degree., and the curves of A-30.degree. and B+30.degree. are the same respective curves.
As seen from FIG. 1(b), the liquid crystal which has the orientation direction of A exhibits, when viewed from above, a reduction in contrast caused by a dark luminance float in an area thereof denoted by arrow mark (a), but exhibits, when viewed from below, a gradation reversal in another area denoted by arrow mark (b), by which the visibility angle characteristic of the liquid crystal is degraded. Meanwhile, the other liquid crystal which has the orientation direction of B has a visibility angle characteristic opposite to that of the liquid crystal which has the orientation direction of A such that a gradation reversal occurs when the liquid crystal is viewed from above and a reduction in contrast occurs when the liquid crystal is viewed from below. For this reason, in a liquid crystal panel wherein a display picture element is divided into two areas of A and B whose visibility angle characteristics when viewed from above and below are opposite to each other, the visibility angle characteristic is given as such an average of the visibility angle characteristics of A and B as denoted at C in FIG. 1(b), and this improves the visibility angle characteristics of the liquid crystal panel when viewed from above and below.
As described above, in an orientation dividing panel, the visibility angle characteristics when viewed from above and below can be improved by dividing a display picture element into two areas of A and B whose visibility angle characteristics when viewed from above and below are opposite to each other. However, in the orientation dividing panel, a boundary line called disclination line (refer to FIG. 2(a)) caused by orientation division is produced in the proximity of a boundary between A and B having different liquid crystal orientation directions, and leakage of light occurs at the location of the disclination line. Consequently, the display quality of the panel is degraded in that the dark luminance becomes lighter to lower the contrast ratio.
Thus, various techniques for intercepting light through a disclination line described above have been proposed.
Japanese Patent Laid-Open Application No. Heisei 5-224210 discloses a technique wherein disclination line 17 is masked with light intercepting film 10 as seen in FIG. 2(a). According to the technique, light intercepting film 10 is provided on opposed electrode substrate 2 of a liquid crystal panel so that disclination line 17 produced by orientation division cannot be observed.
Meanwhile, Japanese Patent Laid-Open Application No. Heisei 5-232474 discloses another technique which makes use of a retention volume line for interception of light through a disclination line. According to the technique, as shown in FIG. 2(b), retention volume line 19 made of Cr or Al is provided in alignment with disclination line 17 on thin film transistor (TFT) substrate 1 of a liquid crystal panel in order to intercept light through disclination line 17. Here, the retention volume line denotes a wiring line for a capacitor added for improvement in voltage holding characteristic.
In the techniques disclosed in the publications mentioned above, however, if a spacer of a plastic material or the like provided to keep the panel gap fixed is located in the proximity of an orientation dividing line, the orientation direction of liquid crystal around the spacer is sometimes so disturbed by the influence of the spacer that the disclination line is displaced from the orientation dividing line on the picture element. Accordingly, the techniques have a problem in that, even if a light intercepting film or a retention volume line is provided for the orientation dividing line, leakage of light through the disclination line cannot be intercepted completely, and this degrades the display quality of the liquid crystal panel.
The displacement between the disclination line and the orientation dividing line by a spacer described above can be eliminated by applying orientation processing to the surface of the spacer so that the spacer may not disturb the orientation direction of the liquid crystal around the spacer. As a technique of applying such orientation processing to the surface of a spacer, a technique is disclosed in Japanese Patent Laid-Open Application No. Showa 57-613 wherein orientation processing is performed for the surface of a spacer to lower the surface energy of the spacer so that the spacer may not disturb the orientation direction of liquid crystal. Meanwhile, Japanese Patent Laid-Open Application No. Heisei 3-69917 discloses another technique wherein a spacer having a surface of a vertical orientation and another spacer having a surface of a horizontal orientation are employed simultaneously so as to reduce the disturbances in orientation by the spacers when a voltage is applied and when it is not applied.
However, the techniques disclosed in the publications just mentioned are directed to improvement in display characteristic of an ordinary liquid crystal panel whose orientation is not divided, and they exhibit little effect on suppression of the displacement between a disclination line and an orientation dividing line described above. In particular, in the technique disclosed in Japanese Patent Laid-Open Application No. Showa 57-613, the surface of a spacer in a liquid crystal panel, in which the surface of a substrate is processed so as to have a vertical orientation and the voltage application portion has a horizontal orientation, has a vertical orientation so as to prevent an irregular orientation, and consequently, the technique does not have an effect of preventing possible displacement of a disclination line for a TN or STN liquid crystal panel in which the voltage application portion has a vertical orientation. Therefore, the technique has a problem in that the displaying quality is still degraded. Meanwhile, in the technique disclosed in Japanese Patent Laid-Open Application No. Heisei 3-69917, as will be described below, a spacer is employed, but since the horizontal orientation processing moves the disclination line by a great amount a large amount of light leaks through the disclination line. Consequently, the technique also has a problem in that the displaying quality is still degraded.